1. Field of the Invention
The invention relates to the formation of a chemical bond between relatively elastic and relatively inelastic materials, for example, between rubber and plastic materials. In particular, the invention relates to an outsole of an article of footwear and methods of making the outsole wherein the outsole comprises both relatively elastic and relatively inelastic materials which are chemically bonded together.
2. Prior Art
Elastic and inelastic materials have been used in the construction of articles of footwear for many years. Particularly, rubber materials have been widely used in the fabrication of midsoles and outsoles of articles of footwear. Rubber and plastic elements frequently are placed in direct communication with one another in the fabrication of an article of footwear with conventional fixation methods including stitching, riveting, screwing, nailing, and the use of various adhesives.
The use of adhesives has numerous disadvantages. Adhesives often are volatile and harmful to the environment both in the emission to the atmosphere of noxious gases and in the emission to the surrounding environment of the adhesives and their by-products. Adhesives also typically do not form a favorable bond between the rubber and plastic and hence, the materials are prone to rip and tear when the adhesive bond fails.
The use of adhesives also often requires the labor intensive steps of roughing at least one of the material surfaces to be joined, applying primer and adhesive, fitting the components, and applying both heat and pressure in order to activate the cement system and form a mechanical bond between the adjoining rubber and plastic materials. This process is time consuming, labor intensive, expensive, and presents numerous opportunities for human, material, or environment variables to adversely influence the quality of the mechanical bonds and associated products. The adhesive bond formed is prone to delamination of plastic and rubber as the result of bond failure induced by fatigue and environmental degradation.
Direct injection of plastic and rubber to one another, to leather, and to many of the natural and synthetic textiles commonly used in the manufacture of articles of footwear also is known. See U.S. Pat. No. 4,481,727 to Stubblefield (double pour mold or double density injection process). Direct injection also has its disadvantages in that it commonly employs heat, pressure, various primers, and the technique of impregnating textiles with the plastic and/or the rubber material in order to enhance the quality of what is nevertheless substantially a mechanical bond formed between the elements. The mechanical bond formed by direct injection also is prone to delamination of plastic and rubber as the result of bond failure induced by fatigue and environmental degradation.
The formation of a chemical (covalent) bond between specific rubber and plastic articles is known and described in U.S. Pat. Nos. 4,816,345 and 4,921,762 to Jadamus et al. U.S. Pat. No. 4,816,345 discloses the bonding of a molded piece of a polyphenylene ether-containing polymer and a molded piece of an elastomeric thermoplastic block copolymer of the type A-B-A, wherein A represents a styrene block and B represents a soft phase block, for example a block of polymerized conjugated diene which may or may not have been selectively hydrogenated. The bonding is achieved by heating of the two pieces while contacting each other. The bonding mechanism is not known, but is distinguished from vulcanization. U.S. Pat. No. 4,921,762 disclose a process for bonding a thermoplastic polyphenylene-ether-containing polymer and a rubber (EPDM and/or styrene-butadiene rubber) by covulcanization, wherein the rubber portion contains peroxidic vulcanization agents and vulcanization activators. Additionally, in the background thereof, it is disclosed as known that SBR and EPR rubbers and polybutadienes have high adhesion to polysulfones, polycarbonates and certain polyphenylene ethers after a thermal treatment.
Further, vulcanization is known is the art as a means of attaching shoe portions to each other. U.S. Pat. No. 4,130,947 to Denu discloses an outsole layer extending across the entire bottom of the shoe which has been vulcanized to a softer midsole layer. U.K. Pat. Specification No. 298,718 to Looms et al. discloses vulcanizing a separate forefoot tread member and a heel tread member to a sole base or foundation. U.S. Pat. No. 4,676,010 to Cheskin discloses a sole having a plurality of discontinuous and independent outsole pieces attached to the lower surface of the midsole by vulcanization. The exposed lower surface of the midsole is asserted as making a significant contribution to the flexibility and weight reduction of the sole. However, because of the chemical make up and the dissimilar properties of rubbers versus thermoplastics and thermoplastic elastomers, vulcanization of one to the other without pretreatment of one or both substrates cannot effectively be achieved.
The aforementioned methods of chemically bonding rubber and plastic together typically utilize expensive machinery, and specific raw materials. In the footwear industry, in particular, manufacturing cost is of paramount concern. Thus, there is a need in the art for reduction of costs of production including improving efficiency of manufacture and decreasing the cost of raw materials.
It is generally known that the use of grooves in the outsole of an article of footwear improves the flexibility of the sole. See U.S. Pat. No. 4,562,651 to Frederick. U.S. Pat. No. 5,012,597 to Thomasson also discloses that the grooves reduce the thickness of the sole bottom at their locations and render the sole easier to bend at the locations of the grooves in the direction perpendicular to the grooves. The formation of grooves in the outsole and/or midsole for an article of footwear typically entails additional manufacturing or processing steps or the use of special molds designed to include the grooves. Thus, there exists a need to provide for enhanced flexibility in an outsole for an article of footwear which does not require the use of grooves.
U.S. Pat. No. 5,024,007 to DuFour discloses that a walking sole for a golf shoe which comprises an outsole comprised of a relatively rigid molded plastic material and an elastic plastic material has improved longitudinal flexibility and a greater degree of transverse rigidity. The elastic plastic material fills notches and elongated openings present in the relatively rigid molded plastic material. Outsoles comprised entirely of plastic materials typically are inappropriate for most articles of footwear, i.e., athletic footwear such as shoes for running, basketball, tennis, racquetball, etc. The excess rigidity, weight and lack of traction render outsoles comprised entirely of plastic materials undesirable for usein most athletic footwear. Outsoles comprised entirely of rubber materials for articles of footwear can lack necessary support.
Thus, there also exists a need to formulate an article of footwear which comprises an outsole having various physical and mechanical properties in different regions, and wherein these regions are adequately bonded to one another. There exists a need to provide an outsole for use in an article of footwear which has greater flexibility, less weight and has sufficient durability. Additionally, there is a need to provide an efficient and economical method for bonding relatively elastic and relatively inelastic materials for use in an article of footwear without utilizing adhesives.